One potential causative factor of diet-induced obesity (DIO) is leptin resistance. Leptin is a potent anorexic hormone in most lean animals. However, the effects of leptin are diminished in obese animals, and this refractiveness is known as leptin resistance. Diets high in fat and sucrose lead to leptin resistance and obesity, yet the causative dietary ingredient(s) remain elusive. Our data demonstrate that a low-fat diet with elevated fructose causes leptin resistance, whereas diets without fructose do not. Leptin signals via the signal transducer and activator of transcription protein 3 (STAT3) and the nutrient sensing AMP-activated kinase (AMPK)/acetyl CoA carboxylase (ACC) pathways. Leptin signals through the latter by promoting dephosphorylation of pAMPK/pACC. We demonstrated that fructose prevents leptin-mediated pAMPK/pACC dephosphorylation, leading to leptin resistance. Over-nutrition also leads to leptin resistance. We suggest that diets that lead to over-consumption, exacerbated by fructose, promote inflammation, endoplasmic reticulum (ER) stress, and reactive oxygen species (ROS) and these specifically inhibit leptin-mediated STAT3 phosphorylation (P-STAT3). We postulate that elevated dietary fructose, but not glucose, impairs the leptin- mediated dephosphorylation of pAMPK/pACC in the hypothalamic arcuate nucleus (ARC) and ventral tegmental area (VTA). Additionally, we postulate that fructose, in synergy with over-nutrition, increases inflammation, ER stress, and ROS, thus inhibiting pSTAT3 signaling. Furthermore, disruptions in both pathways are necessary to promote weight gain. We will test these hypotheses in male Sprague Dawley rats by examining the neural regulation of food intake and body weight, whole body energy homeostasis, and brain region-specific leptin signaling. Exp 1 will examine if 2-week dietary fructose inhibits the leptin-mediated dephosphorylation of pAMPK/pACC or impairs the phosphorylation of STAT3 compared with that of dietary glucose or sucrose. Exp 2 will determine if fructose acts directly on central leptin signaling pathways by central infusion of fructose or glucose. Exp 3 examines if disruptions in both the STAT3 and AMPK leptin signaling pathways are critical for the leptin-resistance promoted weight gain by specific inhibition of the individual pathways. In Exp 4, we will examine if over-nutrition plus fructose (vs. glucose) will induce a greater brain inflammation/ER stress/ROS, and consequently, result in more severe inhibition of leptin-mediated P-STAT3 signaling in the ARC and VTA. Exp 5 will examine the individual contribution of inflammation, ER stress or ROS production to fructose/over-nutrition-induced leptin resistance by infusion of respective inhibitors of these stressors. Our approach is unique because we suggest that fructose plus over-nutrition constitute a two-hit model explaining leptin resistance. An understanding of these mechanisms will provide new therapeutic strategies for obesity, including drugs that target inflammation/ER stress/ROS coupled with low fructose diets to resurrect leptin action. PUBLIC HEALTH RELEVANCE: The American population may be particularly vulnerable to dietary obesity due to chronic consumption of high sugar and fat diets, and obesity-related health complications hasten the onset of disability and death. This proposal seeks to examine the role of dietary fructose and fat in the promoting weight gain. In particular, it examines the way fructose and over-consumption prevents the hormone leptin from its normal action, thus promoting weight and fat gain. Our proposed studies will be conducted in Sprague-Dawley rats, a pre-clinical rodent model for diet-induced obesity in humans.